The present invention relates to a spectrometer for measuring the wavelength of a radiation emitted from a light source which can produce a line spectrum within a limited wavelength range, and more particularly to a spectrometer suited to measure the wavelength of a radiation which is emitted from an excimer laser, with high resolving power and high precision.
An excimer laser is used as a high brightness light source for generating ultraviolet radiation which is used to form the image of ultra fine patterns on a semiconductor substrate, thereby forming a semiconductor integrated circuit. In general, the image of fine patterns having a minimum dimension less than half a micron is formed on the semiconductor substrate by a stepper including a projection lens with high resolving power. Since for ultraviolet radiations are used as the exposure light of the stepper, the material for forming the projection lens is limited to those capable of transmitting ultraviolet radiations, such as quartz. Accordingly, in order to prevent the chromatic aberration due to the projection lens, it is necessary to make the wavelength of the exposure light coincide with the design conditions of the projection lens, and moreover it is required to make the spectral width of the exposure light very narrow. Hence, it is necessary to precisely control the wavelength of a radiation emitted from an excimer laser which is used as the light source of a stepper, and means for measuring the emission spectrum of the excimer laser very precisely is indispensable for such precise wavelength control.
The wavelength of a radiation emitted from a light source which produces a line spectrum, has hitherto been measured by a spectrometer provided with a diffraction grating. In many cases, the above wavelength has been measured by a Czerny-Turner monochrometer, which is described, for example, on pages 1066 and 1067 of a Japanese textbook entitled "Kogaku Gijutsu Handbook (Optical technology Handbook)" and published in 1980.
FIG. 10 shows the optical system of the Czerny-Turner monochrometer. Referring to FIG. 10, a light beam passing through an entrance slit 1 is converted by a concave mirror 2 into parallel rays, which are reflected and diffracted from a plane diffraction grating 4. A light component which is included in the diffracted light and has a specified wavelength, is reflected from another concave mirror 3 and focused on an exit slit 42 by the concave mirror 3, to pass through the exit slit 42. The wavelength of the light component which passes through the exit slit 42 is dependent upon the groove spacing of the grating 4 and the angle of rotation thereof. When the intensity of light passing through the exit slit 42 is measured while rotating the grating 4, the emission spectrum of a light source is obtained, as shown in FIG. 11.
Now, let us suppose that the groove spacing of the grating 4 is 1/3600 mm, and the focal length of the concave mirrors 2 and 3 is 1 m. In this case, the spectral dispersion on the exit slit is, approximately 4 mm/nm for a wavelength in the vicinity of the wavelength 248.4 nm of the laser beam emitted from a KrF excimer laser. Accordingly, it is basically possible to measure the absolute wavelength and spectral width of the radiation which is used as the exposure light of a stepper, with a precision of the order of 0.001 nm.
In a case where an excimer laser is used as the light source of a stepper, it is required to make constant the frequency of the excimer laser, or to make constant the wavelength of the emitted laser radiation in a predetermined medium, that is, in vacuum. In a case where the wavelength of a laser beam is measured in air by a spectrometer provided with a diffraction grating, however, the refractive index of air varies with temperature and atmospheric pressure, and hence the measured wavelength of the laser beam also varies with temperature and atmospheric pressure. Accordingly, in a case where the wavelength of a laser beam is measured in air, the simultaneous measurement of the wavelength of the laser beam and the wavelength of a reference light beam having a constant frequency, by means of the same spectrometer is required, to calibrate the spectrometer and to correct the influence of variation in refractive index of air on the measured wavelength of the laser radiation.
The wavelength of the radiation emitted from a light source which produces a line spectrum, and the fluctuation of the wavelength with time can be measured by a photoelectric detector with high spatial resolution, for example, a linear image sensor or the like. In this case, when the spectral image that is focused by a concave mirror or the like is enlarged by a lens system and the enlarged spectral image is focused on the photo-sensitive surface of the detector, the wavelength of the radiation can be measured, with very high precision. In a case where the difference in wavelength between the radiation to be measured and the reference light is so small that both spectral images can be simultaneously focused on the photo-sensitive surface of a detector, the reference light can be used as the standard for the wavelength of the radiation to be measured. In general, it is difficult to find reference light whose wavelength is nearly equal to the wavelength of the radiation to be measured. For example, a line spectrum which is emitted from a mercury lamp and has a wavelength of 253.7 nm, is suited to be used as the reference light for a radiation which is emitted from KrF excimer laser with a wavelength of 248.4 nm. However, in the above-mentioned spectrometer having a spectral dispersion of about 4 mm/nm, these spectral lines are focused on the focal plane in a state that the spectral lines are, spaced apart from each other with a distance of about 20 mm. When the focused spectral images are enlarged by a lens system and the enlarged spectral images focused on a detector plane, it is impossible to detect these spectral images simultaneously by the detector with limited sensitive area.
In the above example, it is possible to rotate the diffraction grating so that the radiation to be measured and the reference light are alternately received by the same detector. In this case, however, it is impossible to detect both spectra at the same time. Furthermore, an error in the mechanical rotation of the grating will to reduce the accuracy in measurement of wavelength. That is, the prior art has a drawback that it is impossible to measure the wavelength of a desired one of spectral lines existing within a wide wavelength range, with high resolving power, and accuracy.